Joonho So

Full-wave simulation of frequency diverse array antenna using the FDTD method

This paper describes 2-D analysis of a frequency diverse transmit array antenna with a periodic modulated pattern in range using the finite difference time domain methods. The elements of applied linear array are equally spaced and have same phase but operated at different frequencies. The result pattern of frequency diverse array forms range-dependent beam patterns whereas conventional array pattern forms same beam patterns in far field range. This range-dependent beam pattern provides more flexible beam scan options and lessens undesirable interferences such as multipath. The simulation results for radiation patterns of open-ended waveguide (OEG) antenna array are presented. The characteristics of radiation pattern were investigated by the each different simulation for frequency offset change, radiation element space change and array number change.

Empirical analysis of high pressure SF 6 gas breakdown strength in a spark gap switch

It is difficult to predict or measure the breakdown strength of SF6 gas in the high pressure range. Therefore, it is hard to find reference data of the high pressure SF6 gas breakdown strength. This paper attempts to provide empirically a novel criterion for the estimation of SF6 gas breakdown strength in a spark gap switch. For this purpose, J.C. Martins breakdown field strength of non-uniform gap in air and Kuffel et als breakdown field strength by exploiting streamer criterion of uniform field in SF6 gas are adopted. The research is made for the empirical method of predicting the high pressure SF6 breakdown strength and compared results have been ascertained with several experimental data from different investigators in a wide range of pd (pressure times distance) and field enhancement factor (FEF). For further confirmation of the empirical approach, we have designed and developed a high pressure spark gap switch to be closely coupled to a high voltage PFN-Marx system. The designed spark gap switch operates above 1 MV. Our tests have verified the prediction of the breakdown strength of the SF6 filled spark gap switch in a pd range of 0.27-1.62 MPa . cm (2.7 to 16.2 bar.cm).

Transmission of gigawatt-level microwave using a beam-rotating mode converter in a relativistic backward wave oscillator

Gigawatt-level circularly polarized radiation was transmitted using a coaxial beam rotating antenna in an X-band relativistic backward wave oscillator. The mode conversion from the TM01 mode to the circularly polarized TE11-like mode was experimentally and numerically shown. The simulated radiation pattern was in good agreement with the measured radiation pattern.

Theory of Multimode Resonant Backward-Wave Oscillator With an Inclined Electron Beam

A multimode theory of the resonant backward-wave oscillator (BWO) with an electron beam inclined with respect to the surface of a periodic structure-a clinotron-is presented. It is shown that mode interaction provides phase velocity variation in the interaction space. The beam-wave interaction power increases at a favorable phase velocity profile along the interaction space, which manifests as power peaks in the clinotron zone. In contrast, when it is nonfavorable, there is power drop in the bandwidth. Developed multimode theory results are in satisfactory agreement with the theory of a BWO with reflections and with particle-in-cell simulations.

Broadband Microstrip Reflectarray With Five Parallel Dipole Elements

A broadband microstrip reflectarray with five parallel dipole elements is proposed. Based on the operating mechanisms and the reflection phase characteristics of the five-dipole elements, the elements for the broadband reflectarray are designed. The radiation patterns of the five-dipole element reflectarray with the design frequency of f0 = 15 GHz are measured, and 1-dB gain bandwidth of 33.52% is achieved. The measured peak gain at f0 = 15 GHz is 31.4 dBi, which corresponds to the high aperture efficiency of 64.1%.

Mode Conversion of High-Power Electromagnetic Microwave Using Coaxial-Beam Rotating Antenna in Relativistic Backward-Wave Oscillator

Mode conversion of high-power electromagnetic microwave (HPEM) was successfully accomplished using a coaxial-beam rotating antenna (COBRA) in an X-band relativistic backward-wave oscillator (RBWO). The mode conversion from the TM01 mode to the circularly polarized TE11-likeness mode was identified by both cold test and hot test. To observe whether mode conversion is effective or not, the radiation pattern of TE11-likeness mode by COBRA lens was compared with the case of the radiation pattern of TM01 mode using simple horn antenna. Their radiation patterns could be described by fluorescent lamps. The experimental results of radiation pattern measurements are shown to be in a reasonable agreement with the simulated one. The maximum value of RF output power appeared at the center of COBRA lens from the experimental results.

Automodulation Processes in Clinotrons With Low-Focusing Magnetic Field

The automodulation processes in backward-wave oscillator with an inclined electron beam (the clinotron) operating at low-focusing magnetic fields have been studied. The automodulation behavior has been analyzed in the clinotron for different angles of the beam inclination. It has been shown that at low magnetic fields, the RF transverse electric field may cause significant changes in electron trajectories, and hence in the beam-wave interaction power that leads to the automodulation.

A Compact and Wideband Circularly Polarized Rectenna with High Efficiency at X-Band

A new design for a compact and wideband circularly-polarized rectenna with high efficiency operating at X-band is proposed. A dual-slot coupled antenna excited by an H-shaped slot fed by a T-shaped microstrip is designed to yield wideband performance as a receiving array antenna. Rectifying circuit models for harmonic suppression circuit, impedance matching, DC-pass circuit, and DC return circuit at the input and output of the diode are built up and optimized to transfer the maximum power from the antenna to the load using an ADS circuit simulator. An RF-DC conversion efficiency of 71.9% is measured on the conditions of 300 Omega load, and 50.1 mW RF input power at 9.5 GHz operating frequency. For the proposed wideband rectenna, the efficiency of more than 50% is measured over a 1 GHz frequency bandwidth. The measured gain, axial ratio, and return loss of the circularly polarized antenna with a 4-element array are 11.2 dBi, 1.1 dB, and -16.4 dB, respectively. The reflection coefficient of the array antenna is measured at less than -10 dB over a wide frequency range of about 2 GHz. Using this antenna as transmitting (TX) and receiving (RX) radiators, the free-space power transfer capability of the rectenna is tested in free space to turn on an LED at 25 cm distance.

Validation of 3-D Time Domain Particle-in-Cell Simulations for Cold Testing a W-Band Gyrotron Cavity

This paper evaluates the performance and reliability of a commercially available 3-D conformal finite-difference time-domain particle-in-cell (PIC) code, VSim, for cold test simulations of a gyrotron cavity. An interaction cavity for a 95-GHz gyrotron is simulated and optimized using the VSim PIC code to achieve TE6,2 mode excitation. The optimized cavity design is also studied and compared using the commercially available numerical code, CASCADE, and PIC code, MAGIC. A rigorous analysis of the simulation results obtained through VSim and MAGIC is performed using the CASCADE results as a reference. The performance of VSim is also compared with MAGIC based on its accuracy for calculating the resonant frequency and quality factor. Finally, the optimized cavity is fabricated and experimentally tested to measure the resonant frequency and the quality factor. The experimental results confirm the reliability and accuracy of the VSim cold cavity results.

Near-Field Pattern of Large Aperture Higher Order Mode Generator Using Backpropagated Fields in Free Space

This paper reports the estimation of the electric field from a large aperture cavity that generates a higher order mode by backpropagation of fields measured in free space. A higher order mode of a TE6,2 mode at 94 GHz is designed and fabricated using a quasi-optical manner for the analysis. The backpropagating field at the aperture is compared with the measured field. The field estimated at the aperture is used to analyze the mode information, such as the amount of mode mixture of two different rotating fields and the phase difference between two modes. This paper provides a quantitative way to analyze the rotating cylindrical modes generated from an oversized cavity.